It is known that the fire retardancy of polymeric compositions can be improved by the incorporation of halogen compounds therein. Chlorinated materials are among the most common and are usually combined with an inorganic agent such as antimony trioxide. It is also known that certain bromine compounds are effective fire retarding agents and have an advantage, vis-a-vis the chlorine materials, because the same level of fire retardancy can usually be obtained with a smaller amount of additive. Most bromine containing compounds, however, are not stable at the elevated temperatures often encountered in the processing polymeric compositions, e.g., the elevated temperatures required for molding high molecular weight polymers.
Bialous in U.S. Pat. No. 3,673,278 teaches a thermoplastic resin composition asserted to have improved flame retardant and drip control properties which is an admixture of a flame retardant polycarbonate resin and poly(tetrafluoroethylene). The flame retardant aromatic polycarbonates are polymers of a halogen-substituted dihydric phenol such as tetrabromobisphenol-A [2,2-bis-3,5-dibromo-4-hydroxyphenyl(propane)]. Abolins teaches that flame retardant thermoplastic molding compositions can be obtained by incorporating the Bialous admixture with a normally flammable linear polyester and filamentous glass. U.S. Pat. No. 3,334,154 also relates to polycarbonates containing halogenated bisphenols.
General Electric markets one of Bialous' flame retardant aromatic polycarbonate compositions, in particular a polycarbonate prepared from bisphenol-A, tetrabromobisphenol-A and phosgene, as an additive for polybutylene terephthalate injection molding compositions. The brominated polycarbonate additive is used at relatively high levels, about 30%, and if the injection molding is carried out according to the recommended procedure, no problems are encountered. Unfortunately, injection molders often inadvertently wander from the recommended processing conditions, particularly with respect to the processing temperature, which results in a high rejection rate for the final molded product. The tetrabromobisphenol-A polycarbonate melts under such conditions which appears to cause the additive to agglomerate in localized areas instead of being uniformly dispersed throughout the composition. As a consequence, some areas on the product are highly fire retardant while other areas are not fire retardant at all, and particularly with small objects, there is a possibility that the item will contain insufficient of the localized concentration of fire retardant additive and hence be completely flammable.
Other brominated materials have been used for increasing the fire retardancy of thermoplastic compositions used for molding applications. As but one example, reference is made to Schwarz U.S. Pat. No. 3,645,962 which teaches the use of a brominated polyether.
Ismail, in U.S. Pat. No. 3,723,172 teaches that polyesters of an aromatic dicarboxylic acid and a halogenated dihydric phenol (e.g., tetrabromobisphenol-A) can be used as a fire retardant coating for synthetic resinous bodies. Such fire retardant coatings for resinous bodies are susceptible to damage by abrasion and removal by solvents, as compared to inherent fire retardant provided by incorporation of the fire retarding body within the polymers. In addition to being expensive, such coatings are difficult to apply uniformly to a complex molded article.
We have now found that aromatic polyesters of an aromatic dicarboxylic acid and halogenated bisphenols, particularly brominated bisphenols, can be employed as a fire retardant additive and have particular advantageous properties if their glass transition point is above 175.degree. C. This polyester additive can be used in molding compositons and does not melt should the molder inadvertently employ a processing temperature which is somewhat higher than the recommended upper limit. As a result, the additive can eliminate the high rejection rate. Additionally, the polyester additive is non-blooming, i.e. has little or no tendency to migrate to the plastic surface during processing and heat ageing that is often encountered in use. Bloom is a severe problem with monomeric halogen-containing additives. When the additive migrates to the plastic surface, it can be lost from the article as a result of routine abrasion or rubbing which, in turn, causes the object to lose the fire retardant properties for which the additive was used. Blooming also detracts from the appearance of the molded article.
Accordingly, it is the object of this invention to provide a halogencontaining fire retardant additive which is characterized by having little or no tendency to migrate to the surface during molding and in use also has sufficient stability to permit it to be molded with various plastics. This and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention.